MOG is now offering a free version of their music service. Listeners can fill up their “tank” of free music by participating in various “games” to build virality.
I have never used Spotify so I can’t say much about it, but compared to Rhapsody, MOG has a much better user interface. I subscribe to the premium plan, which allows me to listen on my iPhone and download songs to it so that I can listen even when I’m out of 3G or wifi range (e.g., in the subway). MOG also has a service on Roku if you want to listen through your TV.
The selection is pretty extensive, depending on one’s tastes. There are a few prog-rock labels that refuse to go the “music streaming” route. Only iTunes has The Beatles. And there are some jazz artists I have not been able to find on MOG. But if you’re into “popular music,” MOG pretty much has everything.